Follow this link to the Pew Research Center's Religious Landscape study.

If you're unfamiliar with Pew, here's Pew defined right out of Wikipedia. (They are a great resource for well researched data and information on religious and other trends.)

"The Pew Research Center is a nonpartisan American fact tank based in Washington, D.C. It provides information on social issues, public opinion, and demographic trends shaping the United States and the world."